The goal of this project is the exploration of the biology of oncogenic retroviruses and host cells as a strategy for achieving a better understanding of neoplastic change. The project is focussed on viral induction of lymphomas in the bursa of Fabricius as an experimental system, and includes relevant aspects of B-cell development, immunoglobulin (Ig) gene diversification and retroviral replication. Specific aims are: (1) Detection of genes expressed in normal and v-myc induced preneoplastic bursal stem cells (TF cells). Monoclonal antibodies to TF cells will be characterized and additional antibodies developed. TF cell antibodies which recognize normal bursal stem cells will be sought and candidate cell populations identified by such antibodies will be tested for stem cell activity in transplantation assays and for selective sensitivity to transformation by myc. (2) Determination of the roles of helper virus infection and genetic instability in bursal lymphomagenesis. The frequency of progression from v-myc induced preneoplastic transformed follicles (TF) to invasive lymphomas will be determined in the presence and absence of infectious helper virus. The sensitivity of TF cell populations to gamma irradiation will be defined relative to normal stem cell and proliferating cell populations in the bursa. Perturbation of gene conversion at Ig genes in TF cells will be assessed by polymerase chain reaction techniques and a search for abnormal, potentially oncogenic recombination events at Ig loci will be undertaken. (3) Determination of the role of an EGF family-like gene highly expressed in the bursal stroma. The specific stromal cell type expressing this gene will be determined by in situ hybridization. Antibodies will be used to learn if the proteins of this gene are expressed on the surface of bursal stromal cells or are secreted. If so, they will be expressed from recombinant constructs in tissue culture to determine if this gene can enhance the growth of bursal epithelium, normal stem cells or TF cells in culture. (4) Determination of the mechanism of antisense RNA mediated inhibition of retroviral replication. The hypothesis will be tested that antisense RNA can inhibit retroviral replication by interference with the RNA primer for second strand viral DNA synthesis. The structure will be determined of terminally deleted viral DNA molecules formed in the presence of antisense RNA. The ability of antisense RNA to inhibit formation of the second strand primer by RNase H will be tested in an in vitro model system.